1. Field of the Invention
The invention relates to the production of polymerizable silicones comprising siloxy-substituted silane structures. In particular, the invention relates to the production of silanes of high purity having polymerizable groups and substituted with triorganosiloxy groups by substitution of alkoxysilanes with disiloxanes.
2. Description of the Related Art
Polymerizable silicones, such as 3-acryloxypropyltris (trimethylsiloxy)silane, 3-methacryloxypropyltris(trimethylsiloxy)silane, 3-methacryloxypropyltris(dimethylphenylsiloxy)silane, 1-methacryloxymethyltris(trimethylsiloxy)silane, etc. are useful as raw material or monomer for the production of electronic materials, for the production of contact lenses, as raw material in compositions for functional and industrial coatings, polymeric microstructures or cosmetic applications.
The use of polymerizable silicones as raw materials or monomers for the production of electronic materials, for the production of contact lenses and optical components, as raw materials in compositions for functional and industrial coatings, for polymeric microstructures or cosmetic applications, requires the provision of high-purity materials with a reduced content of by-products such as monohydroxysilanes, monoorganoxysilanes and difunctional disiloxanes and also inorganic and metallic impurities.
JP 11-217389 describes a method in which an alkoxy- or aryloxy-substituted silicone (e.g. 3-methacryloxy-propyltrimethoxysilane) is reacted with hexamethyl-disiloxane in the presence of a carboxylic acid (e.g. acetic acid) and an acidic catalyst (trifluoro-methanesulfonic acid), giving a siloxy-substituted product. The method is preferably designed so that hexamethyldisiloxane, carboxylic acid and catalyst are initially introduced before the alkoxy or aryloxy component is added and the reaction is carried out at ca. 20-70° C. By adding a small amount of hexamethyldisilazane (0.0025 mol per mole of alkoxysilane), the reaction is stopped and neutralized, giving a product with reduced purity (<90 area %). The ratio of product G-Si(OSiMe3)3 [G=methacryloxypropyl] to the monoorganoxy by-product G-Si(OMe)(OSiMe3)2 is ca. 8.3 (i.e. ca. 10.7% monoorganoxy by-product). The ratio of product to the difunctional by-product G-[Me3SiO]2Si—O—Si[OSiMe3]2-G is 54.9 (i.e. ca. 1.8% difunctional disiloxane). It is described how through appropriate temperature control the condensation of silanol groups present to difunctional disiloxane is suppressed, but not how the content of monohydroxysilane (silanol) can be limited. The high content of impurities is problematic as regards the aforementioned applications.
From JP 2000-186095 it is known to react hexamethyldisiloxane, a carboxylic acid (acetic acid) and a strong acid (concentrated sulfuric acid) with 3-methacryloxypropyltrimethoxysilane, the latter preferably being added at a temperature of from −10° C. to 0° C. over the course of 30 min. Following the post reaction, washing and distillation, 3-methacryloxy-propyltris(trimethylsiloxysilane) is obtained with a purity of 90.2-97.8%. The product is contaminated with monohydroxy by-product and dimer by-product (difunctional disiloxane). Besides the high content of the specified impurities (in particular difunctional disiloxane), a considerable disadvantage of the method is that following the actual reaction, a lengthy and therefore uneconomical aging of the reaction mixture at various temperatures over a period of 24 h has to be accepted. The aging process is an essential part of the method since during the reaction monohydroxy by-products are formed whose content is reduced in the course of the aging process by converting the silanols into difunctional disiloxane via a dehydrogenation. In this connection, contents of difunctional disiloxane up to 10% are obtained, which is unacceptable for a number of applications.
EP 1510520 A describes the production of high-purity branched siloxanes with a reduced content of monoorganoxysilane or monohydroxysilane impurities in a two-stage process via the post-reaction of a contaminated crude product (following complete or following partial work-up and/or isolation) with disiloxane and acid in high yield. However, the cited method has the disadvantage that, firstly, on account of the aqueous work-up, monohydroxysilane and/or monoorganox impurities can still continue to form in the acidic medium. The after-treatment can reduce the content of monoorganoxysilane or monohydroxysilane contamination. Secondly, the essential disadvantage of the method is that prior to the post-reaction, the crude product must be worked up completely or at least partially and that consequently the washings, phase separations (and optionally also distillations) are to be carried out twice.
EP 1472264 A describes a method for the production of high-purity silicone compounds via the substitution of alkoxy, aryloxy or acyloxy groups by trialkylsiloxy units. In the presence of a carboxylic acid (acetic acid), an acidic catalyst (CF3SO3H), a disiloxane (Me3SiOSiMe3), the reaction is started by adding an alkoxy- or aryloxysilane.
A carboxylic anhydride (e.g. acetic anhydride) is then added. The reaction system remains largely single-phase, which increases the amount of the base to the used for the neutralization.
For example, 3-methacryloxypropyltris(trimethylsiloxysilane) is obtained in high yield with a purity of 98.7%, the impurities obtained being 1.3% disiloxane (dimer) and less than 0.5% of the monoorganoxysilane by-product CH2═CH(CH3)COO—(CH2)3—Si(OMe)(OSiMe3)3. For further products, considerably lower purities (down to 93%) and higher contents of dimeric compounds (up to 4.4%) are measured. However, how the content of monohydroxysilane by-product can be kept low until isolation of the product and how its formation can be prevented in the course of the work-up/neutralization is not described.
Besides the high required amount of carboxylic anhydride, the essential disadvantage of the cited method is that the content of difunctional impurities (disiloxane) is furthermore above 1% and upon dispensing with a distillation, also inorganic and polymeric organic impurities are said to be present. In the case of a distillation, a significantly lower yield would be expected. Especially for the production of electronic and optical media and components, for the production of contact lenses and also for use in functional coatings, a markedly lower content of difunctional impurities is required. In these applications, in particular the content of difunctional silicones in the polymerizable silicone is troublesome since it triggers, by increasing the crosslinking density, a modification of the mechanical and thus also of the optical properties and/or of the desired properties for use in the sector of electronic materials or functional coatings. Residual contents of monohydroxy and monomethoxy impurities can lead in these applications, as a result of cleavage reaction and/or condensation reactions, to an subsequent change in the functional properties of the materials, components or coatings. Residual contents of inorganic salts or polymeric fractions can lead to undesired light scattering and inadequate performance of optical components. In electronic materials, in this case a significant change in the insulation effect/conductivity and the breakdown field strength is to be expected. Surface properties of functional coatings can be lastingly adversely affected by said impurities. It is therefore essential that polymerizable silicones are made available in high-purity form with a minimum content of difunctional silicones, still condensation-reactive silicones and also inorganic impurities.